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We do them on occasion, my mechanic bud does them regular. Many times its easiest to cut the heads off, (grind or usually torch them) rap on the manifold and slide off over the now studs, remove them after the fact. This avoids breaking them off at the head.

the problem is always access. if you can get at the bolts than any combination of heat, cold (like cooling the bolt with electronic contact cleaner when the threaded block is hot), and vibration (like a air hammer with a center punch like tool to put on the center of the hex and just hammer for 30 seconds or so) will do the job. in a shop setting where time is money, they would just torch the heads off and remove the manifold. then they would heat the exposed bolt stems, let them normalize, heat them again, let them normalize, then soak the base of the studs with penetrating oil while they are still hot but not so hot that the oil vaporizes right away. the penetrating oil is heat seeking like soldering flux is when sweating copper pipe. then when you break them, always use a back and forth motion trying for a little more range of motion each time.

Apply penetrating oil, wait several hours, air hammer the head for a bit, then impact driver. If the impact driver won't spin them right out, don't break them just yet. Resort to heating the bolt to a straw yellow color and allow it to cool to a dull red before trying the impact again.

The advantage of trying to spin it off really hot is that the steel is softer and will deform as it encounters inconsistencies such as corrosion in the thread interface. The disadvantage is that since the steel is softer at this temperature, it will shear easier, most likely flush with the head. Drilling and using a screw extractor in that space is not fun at all.

80% of failures are from 20% of causesNever compromise your principles today in the name of furthering them in the future."All I ever wanted was an honest week's pay for an honest day's work." -Sgt. Bilko"We are generally better persuaded by reasons we discover ourselves than by those given to us by others." -Pascal"Since we cannot know all that there is to be known about anything, we ought to know a little about everything." -Pascal

I second using the impact! it will shake loose a bolt that would twist off normally. I used to own a muffler shop and an automotive machine shop/garage. If you use heat then consider how shrinking works when using it.
There isn't much penetrating oil can do on a 3" bolt.
Impact and flank-drive swivel sockets working both directions with good safety gasses and holding your mouth right
You might get a friend to use the impact while you heat the head with the torch if possible also. Just don't do more damage than good.

to break your bolts loose

To break your bolts loose i would just get a socket and ratchet and tack weld a breaker bar to your ratchet to give yourself some leverage. You should do this when the engine is hot because what happens is the engine heats up and cools down which causes the bolts to get cold welded into the threads. Hope this was helpful

Most good techniques have been mentioned assuming you can access the bolts. If there is anyway to spray a penetrating oil near these bolts, it will probably trickle down into the threads. MY FAVORITE IS PB BLASTER

I'll second the PB Blaster. Heat the bolts, tap with a hammer, and give them some PB. Let them cool and go to work.
A 454 doesn't give you a lot of room, but if you have a 3/8 butterfly impact it will serve you well here. They wil rattle the rust loose without shearing off the bolts.